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Fault current on tap changing power transformer 3

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Mbrooke

Electrical
Nov 12, 2012
2,546
How does fault current vary on the secondary of a power transformer (ie 115kv to 13.8kv 40MVA) as the tap changer adjusts up and down in relation to primary side voltage? Does the X/R ratio exhibit any change? Do I need a tester for exact values or can they be adjusted easily by hand in relation to those of the normal neutral position?
 
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The %Z of transformer is affected by the tap position.
This is only slight and generally the tolerance considered on the rated %Z while calculating the fault current will cover this.

Rompicherla Raghunath
 
It depends on the design of transformer. Depending on the geometrical position of regulating winding with respect to other windings, %Z can vary substantially at different taps. Refer transformer factory test report.
 
About all you can say is that impedance, and therefore fault current, changes with change in tap. To know the change you need to have the results of an impedance test at that tap, or that combination of taps with both the no-load tap and the load tap in the desired position during the test. I think I've seen as many as five values, but generally one, on transformers that could have as many a 5 * 33 = 165 different impedances.

Depending on where on the winding the taps in question are connected an increase in ratio between high and low could result in either an increase or a decrease in impedance.

Generally the differences should be small enough to fall into the model's general uncertainty bucket; if the third significant digit is important you probably want to get the impedance tested at that location.

Haven't tried it, but it would seem that if there's a known load that can be switched on and off at different tap settings and you've got really good metering on both sides of the transformer it would seem that you could at least determine whether the impedance went up or down for that change is load. Since the transformer impedance is mostly reactance, a reactive load, cap bank or shunt reactor, would give a better indication than a strictly real power load. It is also possible that the R and X components of the impedance change in opposite directions with the overall change dominated by the change in X.
 
Generally at what tap position is the short circuit current at its highest?


Can I just test that position with an omicron or do I need to test all taps?
 
Depends on the winding design. With a reversing tapchanger the neutral tap has the lowest impedance. With plain taps it may not be the neutral tap - as prc says, 'it depends'.
 
I have reversing in mind.

The nameplate is based in the neutral tap, right?

BTW- do they even make ATC step tap power transformers? I don't think I have ever seen one outside of off load selection.
 
As a general, increased tap means lower secondary for a given primary voltage (as the runs ratio increases). So when you add a small portion of extra winding in the circuit to reduce the secondary voltage. Hence the transformer impedance will be increased and vice versa. At the early stage of my carrier, I took it some time to get digested this intuitively.

In some cases, the increase/decrease will on pro-rata basis. In other cases it is not.

Where the impedance is not on pro-rata basis, one should get the impedances at the extreme tap positions. Then for the intermediate taps, it is the linear variation. Hence increased or decreased based on the tap position.
 
Tappings in transformers is a vast topic and let us confine to the specific point raised by Mbrooke. In the case mentioned, tap changer can be provided on (a) HV to take care of HV variation (b)on LV for LV variation (c) On HV for LV variation (d) On LV for HV variation. You have to make clear which option you are referring to and accordingly the impedance variation will differ. in IEC world, highest tap (say No17) corresponds to lowest voltage. Tap No1 corresponds to the highest varying voltage.

In the above 4 alternatives(there are still further options like with series injection winding etc which are nowadays not common) there can be further 4 variations in each category - (1) tappings on winding (2)separate tap winding outer most (3) tap winding innermost (4) tap winding between HV and LV. In all these 16 alternatives impedance variation with tap changing will be different.

Of course under each, still there are variations-linear tap winding, reversing tap winding, coarse-fine tap windings etc

To avoid confusion, generally in the type mentioned by M brooke, it will be a1 or a2 option for which change in % impedance is negligible. But with (b), constant % impedance change will be negligible, with b1 or b3 option.Remember with constant %z at all taps, fault current will be maximum at lowest voltage tap.(highest tap number)

Sometimes customer may ask constant fault current at all taps(ie constant ohmic impedance -not %impedance-at all taps) - a requirement in olden days- for which also transformer designers have solutions!
 
Thank you Scotty ! So it is from IEEE world with LV tappings for LV variation with preventive auto-transformer. Normally in such designs tappings will be innermost ie constant % impedance at all taps ie highest fault current on lowest tap. You can check this by conducting a low voltage test. Short LV and give LV voltage from HV .From this you can roughly estimate %Z at rated and extreme taps.
 
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